Integrated fluxgate magnetic sensor and excitation circuitry

Abstract

Improved magnetic sensor excitation circuitry is presented for providing a periodic bidirectional excitation waveform to a fluxgate magnetic sensor excitation coil using a bridge circuit connected to the excitation coil and having lower transistors for switched selective connection to a current mirror input transistor to mirror a current provided by pulsed current source, and with integrated filtering to control pulse rise times and slew rate.

What is claimed is: 
     
       1. An excitation circuit for driving an excitation coil of a fluxgate magnetic sensor, the excitation circuit comprising:
 a first transistor coupled between a first power supply node and a first end of the excitation coil; 
 a second transistor coupled between the first power supply node and a second end of the excitation coil; 
 a pulsed current source with a first terminal connected to the first power supply node and a second terminal alternately providing a first level of current and a second higher level of current; 
 a current mirror circuit, including:
 an input transistor connected between the second terminal of the pulsed current source and a second power supply node, 
 a third transistor connected between the first end of the excitation coil and the second power supply node, and 
 a fourth transistor connected between the second end of the excitation coil and the second power supply node; and 
 
 a control circuit operative to alternately couple one of the third and fourth transistors with the input transistor to mirror the second level of current from the pulsed current source to provide a periodic bidirectional excitation current waveform to the excitation coil. 
 
     
     
       2. The excitation circuit of  claim 1 , wherein the control circuit is configured to:
 (i) turn the first and second transistors on and turn the third and fourth transistors off when the pulsed current source is providing the first level of current; 
 (ii) then turn the first transistor on, turn the second transistor off, and couple a control terminal of the input transistor with a control terminal of the fourth transistor when the pulsed current source is providing the second higher level of current to form a first current mirror to conduct excitation current of a first polarity through the excitation coil; 
 (iii) then turn the first and second transistors on and turn the third and fourth transistors off when the pulsed current source is again providing the first level of current; 
 (iv) then turn the second transistor on, turn the first transistor off, and couple the control terminal of the input transistor with a control terminal of the third transistor when the pulsed current source is again providing the second higher level of current to form a second current mirror to conduct excitation current of a second opposite polarity through the excitation coil; and 
 (v) repeat (i) through (iv) to provide the periodic bidirectional excitation current waveform to the excitation coil. 
 
     
     
       3. The excitation circuit of  claim 2 , wherein the control circuit operates the pulsed current source to alternately provide the first and second levels of current. 
     
     
       4. The excitation circuit of  claim 2 , comprising an integrated filter circuit to control rise times of the excitation current waveform provided to the excitation coil. 
     
     
       5. The excitation circuit of  claim 4 , wherein the integrated filter circuit includes a resistance coupled between the control terminal of the input transistor and the control terminals of the third and fourth transistors. 
     
     
       6. The excitation circuit of  claim 2 , wherein the second power supply node is a circuit ground, wherein the first power supply node is a positive voltage node, wherein the first and second transistors are PMOS devices, and wherein the input transistor and the third and fourth transistors are NMOS devices. 
     
     
       7. The excitation circuit of  claim 2 , wherein the first power supply node is a circuit ground, wherein the second power supply node is a positive voltage node, wherein the first and second transistors are NMOS devices, and wherein the input transistor and the third and fourth transistors are PMOS devices. 
     
     
       8. The excitation circuit of  claim 2 , comprising a first switch coupled between the control terminal of the input transistor and the control terminal of the third transistor; and a second switch coupled between the control terminal of the input transistor and the control terminal of the fourth transistor;
 wherein the control circuit selectively operates the first and second switches to alternately couple one of the third and fourth transistors with the input transistor to mirror the second level of current from the pulsed current source to provide the periodic bidirectional excitation current waveform to the excitation coil. 
 
     
     
       9. The excitation circuit of  claim 2 , comprising a resistance connected between the control terminal of the input transistor and a node joining the first and second switches. 
     
     
       10. The excitation circuit of  claim 1 , wherein the control circuit operates the pulsed current source to alternately provide the first and second levels of current. 
     
     
       11. The excitation circuit of  claim 1 , comprising an integrated filter circuit to control rise times of the excitation current waveform provided to the excitation coil. 
     
     
       12. The excitation circuit of  claim 11 , wherein the integrated filter circuit includes a resistance coupled between a control terminal of the input transistor and control terminals of the third and fourth transistors. 
     
     
       13. The excitation circuit of  claim 1 , comprising a first switch coupled between a control terminal of the input transistor and a control terminal of the third transistor; and a second switch coupled between the control terminal of the input transistor and a control terminal of the fourth transistor;
 wherein the control circuit selectively operates the first and second switches to alternately couple one of the third and fourth transistors with the input transistor to mirror the second level of current from the pulsed current source to provide the periodic bidirectional excitation current waveform to the excitation coil. 
 
     
     
       14. The excitation circuit of  claim 13 , comprising a resistance connected between the control terminal of the input transistor and a node joining the first and second switches. 
     
     
       15. The excitation circuit of  claim 1 , wherein the first and second transistors, the pulsed current source, the current mirror circuit and the control circuit are implemented on or in a single integrated circuit. 
     
     
       16. The excitation circuit of  claim 1 , wherein the first level of current provided by the pulsed current source is zero. 
     
     
       17. A magnetic sensing apparatus, comprising:
 a fluxgate sensor structure, including a core structure, at least one excitation coil wound around at least a portion of the core structure, and a sense coil; 
 an excitation circuit, including:
 a first transistor coupled between a first power supply node and a first end of the excitation coil, 
 a second transistor coupled between the first power supply node and a second end of the excitation coil, 
 a pulsed current source with a first terminal connected to the first power supply node and a second terminal alternately providing a first level of current and a second higher level of current, 
 a current mirror circuit, including an input transistor connected between the second terminal of the pulsed current source and a second power supply node, a third transistor connected between the first end of the excitation coil and the second power supply node, and a fourth transistor connected between the second end of the excitation coil and the second power supply node, and 
 a control circuit operative to alternately couple one of the third and fourth transistors with the input transistor to mirror the second level of current from the pulsed current source to provide a periodic bidirectional excitation current waveform to the excitation coil; and 
 
 a sense circuit coupled with the sense coil and operative to demodulate a sense signal from the sense coil to provide an output indicative of an external magnetic field in which the magnetic sensing apparatus is located; 
 wherein the control circuit synchronizes operation of the sense circuit and the excitation circuit. 
 
     
     
       18. The magnetic sensing apparatus of  claim 17 , wherein the control circuit is configured to:
 (i) turn the first and second transistors on and turn the third and fourth transistors off when the pulsed current source is providing the first level of current; 
 (ii) then turn the first transistor on, turn the second transistor off, and couple a control terminal of the input transistor with a control terminal of the fourth transistor when the pulsed current source is providing the second higher level of current to form a first current mirror to conduct excitation current of a first polarity through the excitation coil; 
 (iii) then turn the first and second transistors on and turn the third and fourth transistors off when the pulsed current source is again providing the first level of current; 
 (iv) then turn the second transistor on, turn the first transistor off, and couple the control terminal of the input transistor with a control terminal of the third transistor when the pulsed current source is again providing the second higher level of current to form a second current mirror to conduct excitation current of a second opposite polarity through the excitation coil; and 
 (v) repeat (i) through (iv) to provide the periodic bidirectional excitation current waveform to the excitation coil. 
 
     
     
       19. An excitation circuit for providing a periodic bidirectional excitation current waveform to a magnetic sensor excitation coil, the excitation circuit comprising:
 a pulsed current source with a first terminal connected to a first supply node and a second terminal alternately providing a first level of current and a second higher level of current; 
 a bridge circuit with first and second branches individually including upper and lower transistors coupled in series with one another between the first supply node and a second supply node and joined to one another at a corresponding internal node, the corresponding internal nodes each individually connected to a corresponding end of the excitation coil; and 
 a switched current mirror circuit, including:
 an input transistor connected between the second terminal of the pulsed current source and the second supply node, 
 first and second switches individually coupled between a control terminal of the input transistor and a control terminal of a corresponding one of the lower transistors of the bridge circuit branches; and 
 
 a control circuit configured to selectively operate the first and second switches to alternately couple one of the lower transistors with the input transistor to mirror the second level of current from the pulsed current source to provide a periodic bidirectional excitation current waveform through the bridge circuit to the excitation coil. 
 
     
     
       20. The excitation circuit of  claim 19 , comprising an integrated filter circuit to control rise times of the excitation current waveform provided to the excitation coil.